1. Field of the Invention
The present invention relates to a frequency-variable-type filter, an antenna duplexer, and a communication apparatus, for example, which are used in the microwave band.
2. Description of the Related Art
In a conventionally-known frequency-variable-type filter, a resonator is connected to a switching device such as a PIN diode or a variable capacitance diode through a capacitor or the like to perform voltage control thereof so as to vary a resonance frequency (cf. Japanese Unexamined Patent Publication No. 7-321509). When a PIN diode is used, a frequency is switched by switching it on/off, so that two bands including a band in the on-period and a band in the off-period are provided. In the on-period, usually, a positive control voltage is supplied in order to switch the PIN diode on, whereas a negative voltage is supplied in the off-period. The reason why the negative voltage is necessary in the off-period is that when high-frequency signals of large electric power are input, a high-frequency voltage is applied to the PIN diode and it is switched on, which should be avoided. In other words, when a large amount of electric power is input, the PIN diode becomes unstable and thereby the frequency characteristic varies. Consequently, this needs to be avoided.
However, in the conventional frequency-variable-type filter, in order to switch a PIN diode off, a power-supply circuit for generating a negative voltage (approximately -3 through -10 V) is required, so that the circuitry is complicated, leading to an obstacle to miniaturization and cost reduction of a mobile phone or the like.